322 METEOROLOGY AND ALLIED SUBJECTS. 



Tliey bare also adopted the opinion that the aurora may exist at the 

 height of only a few thousand feet. [Z. 0. G. M., XV, 1880, p. 415.) 



Dr. A. Wijkander, of Lund, Sweden, in a discourse at Stockholm be- 

 fore the Scandinavian Association of "Naturalists, reviews the progress 

 of our knowledge during the past twenty years, in reference to the mag- 

 netic phenomena of the Arctic zone. 



Lefroy, in 1855, showed that at Lake Athabasca and Fort Simpson, 

 the easterlj^ perturbations of the needle during the morning and the 

 westerly perturbations of the evening are the largest, whereas the oppo- 

 site is the case at Toronto and southern stations ; Sabine subsequently 

 found the same results for Point Barrow and Toronto, Canada. 



Wijkander, as the result of the observations of the Swedish Station at 

 Spitzbergen, concludes that, as a general rule, the belt where the auroras 

 are most frequent is the limit between the regions where the maximum 

 of easterly perturbations in declination in the morning and that of the 

 westerly perturbations in the evening occnr together, and the region 

 where these maxima occur in the evening and morning respectively. 



Concerning the annual variations of the perturbations, very little is 

 as yet known. It would, however, appear that the easterly perturba- 

 tions predominate. The perturbations of the horizontal intensity afford 

 little basis for general conclusion on account of their scantiness at Lake 

 Athabasca and Fort Simpson. If we compare the perturbations of 

 declination and horizontal intensity among themselves, we notice that 

 the time of their greatest number and magnitude alternate, a circum- 

 stance to which Wyprecht has also called attention. According to 

 Wijkander, there is evidence thi^t the total intensity is subject not only 

 to a diurnal change, but also frequently to greater perturbations. {Z. 

 0. G. M., XV, 1880, p. 305.) 



XI. — OPTICAL PHENOMENA. 



Schmidt gives an account of a double horizontal rainbow. This was 

 seen on a beautiful, cloudless day, before the morning mist had entirely 

 disappeared, and was apparently formed near the surface of the water 

 in a small lake near Pilseu. The bow had the apparent form of hyper- 

 bola or parabola, whose axes were determined by the vertical plane 

 passing through the observer and the sun. The apices of these hyper- 

 bolas were turned toward the observer, and the apex of the inner curve 

 was about 12 meters from him and 4 meters below the eye, and at the 

 level of the water of the lake, while the apex of the outer curve was 

 beneath the land. The rainbow colors were all clearly seen. The 

 exi)lanation of this phenomenon depends upon the observed fact that 

 the ordinary rainbow can be considered as a complete circle, if we as- 

 sume that the aqueous particles are observed from a close proximity* 

 In the present case, therefore, we have observed the lower part of such 

 a circular rainbow, which, being projected upon the horizontal water 

 suiface, appears as a hyperbola. {Z. 0. G. AL, XIV, 1879, p. 20.) 



